What is my girl doing? (lil revving motor sound)

[Fowl ball]

Hello fellow chicken lovers! I am still new to this (less than a year) and was wondering what my ladies are doing when they get into a "trance" and make 'little revving motor sounds'? They each have a slight variation, but it is basically the same behavior. They can do it for a few seconds or sometimes long 'zoned out' periods. It's hard to distract them or snap them out of it, unless you physically touch them. It is of no consequence and it has it's endearing moments sometimes, but it has my sweetie and myself both puzzled. Any ideas? It strikes me as something that you might all have see often.

 

[Catalina]

Does it sound sort of like a cat purring?
Mine do this when they're especially happy or eating something really tasty.

 

[ksacres]

my chickens trill to each other when they are settling down for the night. to me, it's the sound of contentment. baby chicks do it too, after they are a couple days old.

 

[NewGuineaChooks]

In addition to the suggestions already mentioned, it may be a predator warning sound. ? It's a little shriller than a happy sound. Chickens will freeze, looking in a particular direction, and trill when they have seen something dangerous. (such as a bird or airplane overhead, a dog going by the fence, or the spray bottle in my hand!) If that's what they're doing, they''ll hold still until they've determined the predator isn't coming to get them.